consistent VM hang during reboot

I am trying to solve a problem with amd64 FreeBSD virtual machines running on a Linux+KVM hypervisor. To be honest I'm not sure if the problem is in FreeBSD or the hypervisor, but I'm trying to rule out the OS first.

The _second_ time FreeBSD boots in a virtual machine with more than one core, the boot hangs just before the kernel would normally print e.g. "SMP: AP CPU #1 Launched!" (The last line on the console is "usbus0: 12Mbps Full Speed USB v1.0", but the problem persists even without USB). The VM will boot fine a first time, but running either "shutdown -r now" OR "reboot" will lead to a hung second boot. Stopping and starting the host qemu-kvm process is the only way to continue.

The problem seems to be triggered by something in the SMP portion of cpu_reset() (from sys/amd64/amd64/vm_machdep.c). If I hit the virtual "reset" button the next boot is fine. If I have 'kern.smp.disabled="1"' set for the initial boot then subsequent boots are fine (but I can only use one CPU core, of course). However, if I boot normally the first time then set 'kern.smp.disabled="1"' for the second (re)boot, the problem is triggered. Apparently something in the shutdown code is "poisoning the well" for the next boot.

The problem is present in FreeBSD 8.4, 9.2, 10.0 and 11-CURRENT as of yesterday.

This (heavy-handed and wrong) patch (to HEAD) lets me avoid the issue:

--- sys/amd64/amd64/vm_machdep.c.orig	2014-05-07 13:19:07.400981580 -0600
+++ sys/amd64/amd64/vm_machdep.c	2014-05-07 17:02:52.416783795 -0600
@@ -593,7 +593,7 @@
 void
 cpu_reset()
 {
-#ifdef SMP
+#if 0
 	cpuset_t map;
 	u_int cnt;

I've tried skipping or disabling smaller chunks of code within the #if block but haven't found a consistent winner yet.

I'm hoping the list will have suggestions on how I can further narrow down the problem, or theories on what might be going on.

Thanks!

JN

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